Karel Hartman University of Victoria Victoria, B.C. Canada. V8W 2Y2
A Student Operated Animated lnfrared Spectroscopy Teaching Model
Although many students are fairly proficient in the interpretation of the main hands in infrared spectra of organic molecules after they have finished an introductory course in organic chemistry, their knowledge and understanding of how the chemical structure of a compound is uniquely related to its infrared spectrum and how the spectrum is obtained technically, often leaves much to he desired. This frequently is the result of the inconvenience encountered if one wants to visualize the complexity of molecular motions and instrument design during lecture hours. Reading on either topic seems to he unappealing to many students. T o help students understand the background of infrared spectroscopy better, the teaching model shown in Figure 1 and described in this article was designed and built a t moderate expense. Students have shown a keen interest in this teaching model during the first year of its existence and hopefully, in addition to its proven value as an entertaining display model, it has filled some of the gaps in their background in infrared spectroscopy. The teaching model features five areas A, B, C, D, and E (Fig. 2). A) a brief introduction to infrared spectroscopy and the eom-
pound used for the spectrum
B) an attractive simple diagram of an infrared spectrophotometer
C) a recorded infrared spectrum of benzocaine D) a set of 12 animated molecular motions E) a set of 1 2 switches to activate area D The model consists of a trapezoid %-in. plywood box with a 2 X 6-ft base and a removable rear panel. The front top part D of the box is divided into 12 one square foot areas. One molecular motion is mounted onto each one square foot area. The front lower parts A, B, C, and E are recessed 3 in. and covered with a transparent colorless sheet of Plexiglas 'h in. thick. The hack of the recessed lower parts consists of a translucent sheet of white Plexiglas which is illuminated from behind by a fluorescent light tube over the width of areas B and C. The 12 animated molecular motions are individually activated by a set of 12 springloaded switches mounted below the spectrum in area E. A projected view with approximate dimensions is given in Figure 3. Area A
The compound ethyl p-aminobenzoate known as benzocaine is used in the model because it ~ r o u i d e hoth s ali~hatic and aromatic character in addition to a number of functional groups and different atoms. Also the recent use of henzocaine in suntan lotions (Solarcaine", Americaine") makes the compound more relevant and appealing to students. The text for this part of the model is given in Figure 4. A sample of benzocaine in a vial is shown. A Fisher-Hirschfelder-Taylor molecular model is mounted here to give the
Figure 3. Trapezoid plywood box with approximate dimensions in inches.
Figure 1. Infrared spectroscopy teaching model.
Figure 2. Layout of ir spectroscopy teaching model featuring introduction (A). instrument diagram (4. spechum (0. animated motions. (D), and control panel (0.
Figure 4. Text and mounling of Me inhcduclion in area A
Volume 53. Number 2,February 1976 / 111
for the Table 1. Band Frequencies and k i g n m e n ~ , b Infrared Spectrum of Ethyl p-Aminobenzoate Frequency
Band
4
5 6 7 8 9 10
11 12
(cm-'1
169s 1610 1588 1488 1457 i380 1185 1120 855
Assignment
rtretchiii bending stretching (ring breathing) rymmetrical bending (cH, rcisrorlng) arvmmetrical bend in^ ICH.1 symmetrical bendingidH,j 6 C-H asymmetrical bending (CH, rocking1 u C--0 ~tretching n C-H out of plane bending (wagging) Y C=O
6 N-H v C-=C 6 C-H 6 C-H 6 c-H
OBellamy, N.. "The Infrared spectra of complex Molecules;' 2nd. ~ d . Methuen . & co. Ltd.. London. England. 1958. b&oss, A . D.. -introa;ction to bracticai infrared spectrorcopy.rz 2nd. Ed.. Butterworth & Co. (Publishers) Ltd.. London, England, 1964. Figure 5. Text and mounting of the inshument diagram for the infrared spechophotometer in area B.
reader an exact size relation to reality. The colors chosen t o represent the different elements are black for carbon (C), orange for hydrogen (H), light blue for oxygen (01, and dark blue for nitrogen (N). These are the colors used in the F-H-T molecular model and for the groups of atoms in area D. The vial and molecular model are mounted on the white cardboard background with Nylon mono filament. Area B
This area covers the instrument diagram as shown in Figure 5. The mirrors are made from sheet aluminum coated with a piece of adhesive glossy aluminum foil. The hot source A and detector L are made from orange and black Plexiglas rod, respectively. Prism G and the rotating sector mirror D are made from clear Plexiglas with D half coated with glossy aluminum foil. The light path through the spectrophotometer is cut out of the white cardboard background on which the optical diagram is drawn. A sheet of transparent orange material (3M Brand Education color transparency film negative, red) is glued on the back of the cardboard behind the optical diagram. The optical light path now shows up very nicely when the fluorescent light in the model is switched on. The glow bar A even gives the impression of being red hot because of the light transmitting power of the orange fluorescent oiece of Plexielas. ~ con&ining the benzocaine sample is made The K B disk from a oiece of translucent ~olvethvlene sheet (weighing . . . boat) stick into the cardboard. The wedge C is made from a triangular piece of aluminum sheet with a narrow base and is also stuck into the cardboard.
Figure 7. Aluminum mount with set screw (%o X %in.) for Styrofoam balls. All dimensions ere given in inches.
convenient spectrum dimensions, 134 X 350 mm. The spectrum was then blown up to 9 X 24 in. on Mylar drafting film (Fig. 6) and mounted on the translucent white Plexiglas sheet with double sided adhesive tape. The 12 bands chosen for the animation in area D are shaded t o make them stand out from the rest of the spectrum. The 12 bands are numbered 1-12 to correlate them with the 12 numbered molecular motions and the 12 switches. Table 1 gives t h e chosen frequencies and the assigned molecular motions.
Area C
Area D
This area contains a blown up infrared spectrum of ethyl-p-aminobenzoate. The spectrum can be recorded on any regular infrared spectrophotometer although the author used a Beckman ir 20 because of its single range and
This area consists of a 2 X 6-ft sheet of arborite. The arborite is divided into 12 areas of 12 X 12 in., routed in the top coating of the arborite to expose the dark brown substrate. Commercially available molded hard Styrofoam balls are used to represent the atoms. The balls are glued on aluminum mounts (Fig. 7) with a domestic type two component slow curing epoxy glue and painted with latex paint in the same colors as the F-H-T molecular model. Table 2 gives the atomic diameters and bondlengths and the dimensions chosen for the Stvrofoam balls. the distances, and the strokes. Some of tge dimensions had to be sacrificed to give the movements sufficient visibility. Also for this reason the frequency of all displayed molecular motions after considerable testing, was set a t 200 strokeslmin. The Styrofoam balls are mounted on the drive rnechanisms behind the area D by rods (welding rod K g in. diame-
Figure 6. Blown up infrared spectrum of ethyl-paminobenzoate (benrocaine).
112 / Journal of Chemical Education
1
i
I
@
~ ~ T ~ O W A W
O H
@ C
@ N
a0
Figure 8. Summary of molecular motions displayed in area D. T = 106': U = 109'. V = 2% in.; W = 3 in.: X = 2% in.; Y = 3% in.; Z = 3% in.
-...
I Figure 10. Mounted drive mechanisms for motions number 10 and 12
Figure 9. Mounted drive mechanism fw motion number 6. "Breathing of the aromatic benzene ring."
ter) through either slots or holes in the arboritetplywood laminate. Each Styrofoam ball is pushed halfway onto the rod before being fixed in place by the set screw in the aluminum mount. Because the four atoms of the CH3 group in motions number 8 and 9 are not planar, the three H-atoms should be mounted hieher on their rods than the stationarv C-atom t o give a representable image of the three dimensional relationshin between the four atoms. All drive mechanisms are built around small quiet gear motors.' The mechanisms are made from small machined brass and steel parts and standard meccano parts. A summary of the molecular motions is given in Figure 8. The heavy black arrows indicate the simultaneous direction of the back and forth motion of the balls. The H-atom in motion number 12 moves in and out of the plane of the board with a stroke o f f 5 in. Only three of the 12 drive mechanisms are shown in Figures 9 and 10. Figure 9 shows the drive mechanism for the umbrella type movement of motion number 6, the so-called breathing of the aromatic benzene ring. Figure 10 shows the drive mechanisms for motions number 10 and 12. All motor mounts are made from single pieces of bent aluminum sheet. Notice that a varietv of two or three ooint svmmetrical or asymmetrical movements can be constructed by connectine oieces of rod in different wavs and usine a simple swiveljoint as shown in Figure 11. This swivel j&t can be used in different planes to give either stretching or bending or out of plane movement. All swivel joints and
-1
4
Figure 11. Simple swivel joint to obtain reciprocating movement off an eccentric drive. Table 2.
Dimensions Chosen for Styrofoam Balls, Representing Atoms,-Bondlengths and Bondnrokes Atomic diametefl (covalent1 (A1
C-H
(in
Styrofoam ball diameter (in.)
Bondlengths lAlb
Styrofoam ball distance (center to center1 (in.)
Styrofoam ball stroke (in.)
1.08
2 314
t 1/2
aGarainer. M. L.. "Electron DistriDution and Propartier of tne Elements:' Science Guide. McHenrv. ~. Illinois 1966. b ~ e a r t R. ; C.. W a n d b o . 0 ~of Cnemistry and Physics," 53rd Ed.. CRC Press.
other moving joints are liberally lubricated with grease to reduce noise and friction. Because most parts of the above described model were built in campus trade shops, the author has given the dimensions in inches and feet rather than millimeters. Acknowledgment
'VWII Gear motor, 200 rpm, 0.675 in. stack, 0.93 A, 115 V, clockwise rotation. Price about $10 each. Von Weise Gear Company, 9353 Watson Industrial Park,St. Louis, Missouri, 63126
The author wishes to express his sincere appreciation to Mr. Harry 'l'owner who designed and enth~raiasticnllyhuilt the 12 drive mechanisms used in this teaching model.
Volum 53,Number2. February 1976 / H 3
